Method of removing sulfur dioxide, nitrogen dioxide and carbon dioxide from gases

ABSTRACT

METHOD OF REMOVING SULFUR DIOXIDE AND/OR NITROGEN DIOXIDE AND/OR CARBON DIOXIDE FROM GASEOUS MIXTURES COMPRISING CONTACTING SAID GASEOUS MIXTURE FIRST WITH A MOLTEN NITRATE SELECTED FROM THE GROUP CONSISTING OF SODIUM NITRATE, SOLVER NITRATE AND POTASSIUM NITRATE AND THEN WITH A MOLTEN HYDROXIDE SELECTED FROM THE GROUP CONSISTING OF SODIUM HYDROXIDE AND POTASSIUM HYDROXIDE OR WITH A MOLTEN MIXTURE OF AT LEAST ONE OF SAID NITRATES AND HYDROXIDES.

United States Patent ABSTRACT OF THE DISCLOSURE Method of removingsulfur dioxide and/or nitrogen dioxide and/or carbon dioxide fromgaseous mixtures comprising contacting said gaseous mixture first with amolten nitrate selected from the group consisting of sodium nitrate,silver nitrate and potassium nitrate and then With a molten hydroxideselected from the group consisting of sodium hydroxide and potassiumhydroxide or with a molten mixture of at least one of said nitrates andhydroxides.

Although the hazards to health and injuries to aesthetics resulting fromair pollution have long been recognized, it has only been in very recentyears that efforts on anything other than a small scale have beenundertaken.

However, with growing local, state, and federal govern ment concern withair pollution, industry is, and increasingly will be, seeking effectiveand economically practical methods forremoving constituents from gaseousproducts which are expelled into the atmosphere.

Two toxic gases which are frequently produced as unwanted by products inthe refining of metals or in the manufacture of glass or metal productsand which can cause considerable irritation are sulfur dioxide (S0 andnitrogen dioxide (NO )..Hence, for example, S0 is released in theroasting of copper sulfide ores and in the burning of sulfur-containingcoals whereas N0 is expelled in glass manufacturing operations whennitrates are employed as batch materials.

This invention, then, has for its primary objective the treating ofthese gases in a manner to remove them from gaseous effluent beingreleased into the atmosphere.

We have discovered that S0 can be removed from a stream'of gas bypassing it through a bath of molten nitrate selected from the groupconsisting of potassium nitrate (KNO silver nitrate (AgNO and sodiumnitrate (NaNO and N0 can be removed by passing the gas through a bath ofa molten hydroxide selected from the group consisting of potassiumhydroxide (KOH) and sodium hydroxide (NaOH). Therefore, to remove thetwo toxic gases in combination with each other, the eflluent would befirst bubbled through the bath of molten nitrate and then through thebath of molten hydroxide or through a single bath comprising acombination of said nitrate and hydroxide.

The reaction of S0 with the molten nitrate to yield a sulfate, as isrecorded below, is very rapid:

wherein M=K, Ag, and/ or Na. The M 80 settles out of the molten salt andis compatible therewith.

The reaction of N0 with the molten hydroxide to yield a nitrite, as isrecorded below, is likewise very rapid:

where M:K and/ or Na. The MNO and MNO arecompatible with the bath.

Passing the gas through molten KOH and/ or NaOH would have the secondaryeffect of removing any carbon dioxide (CO therefrom:

which reaction is also very rapid.

In the following illustrative examples of this invention, reagent gradeNaNO KNO AgNO NaOH, and KOH were utilized which were air dried at 130 C.prior to being used. Anhydrous grade S0 and N0 were used which werelikewise dried [by passing through a column of Mg(ClO prior tointroduction into the reaction vessel. The pure hydroxide melts werefused in zirconium reaction vessels whereas alumina (A1 0 reactionvessels were employed to hold the molten nitrates or the combination ofnitrate and hydroxide.

The reactions were followed by examining the infrared spectra (gas cell)of the gaseous reaction products and the quenched melts (KBr technique)before, during, and

after the' reaction. Analyses of the solid spectra were achieved bycomparison with a collection of control spec' tra of pure compounds.

To better monitor the reactions, inasmuch as the rates thereof areextremely rapid, an atmosphere of argon was maintained above the molten'melt at all times, and the S0 and N0 swept into the reaction vesselwith argon. The flow rates of the gases were held at about 500cc./minute when measured at melt temperature. The reaction vessels wereplaced in a closed system so that the effluent gases could be collectedin bulbs attached to a vacuum rack.

About grams (0.6 mole) of NaNO were melted and maintained at 355 C. S0at a flow rateot about cc./minute at the melt temperature and at apartial pressure of about one atmosphere was bubbled through the melt.Copious quantities of N0 gas were almost immediately recognized by thebrown color thereof and confirmed in the infrared spectrum. The reactionwas continued for two hours to check the stoichiometry thereof byallowing an excess of S0 to react completely with the NaNO A comparisonof the infrared spectrum of Na SO with that of the solid reactionproduct indicated that Na SO constituted the reaction product. Basedupon the known amount of NaNO;, starting material and the resultingweight change between the NaNO and the Na SO, reaction product, it wascalculated that one mole of Na SO was formed from two moles of NaNO inthe melt.

About 50 grams (about 0.5 mole) of KNO were melted and the temperatureof the reaction vessel maintained at 500 C. Again, S0 at a flow rate ofabout 120 cc./minute and a partial pressure of about one atmosphere waspassed into the melt for about two hours. Visual inspection and infraredanalysis of the gas expelled from the melt almost immediately indicatedN0 This reaction was likewise continued to check the stoichiometrythereof reacting completely the KNO with an excess of S0 Infraredanalysis of the reaction prodnot demonstrated it to be K 80 and theresulting weight change led to the calculation that one mole of K 80 wasformed from two moles of KNO in the melt.

1 About 50 grams (about 0.9 mole) of KOH was melted and maintained at440 C. N0 at a flow rate of coloration. Infrared analysis indicated thisbrown gas to be N02.

"A calculation based upon the quantity of N passed through the melt inthe two-hour period, i.e., the amount of N0 required to react with 50grams of KOH, determined that about 0.96 mole of N0 were needed to reactcompletely with about 0.9- mole of KOH; hence, about a 1:1 molar ratio.The substances in the melt after completion of the reaction wereidentified by infrared analysis to be nitrate and nitrite. An aqueoussolution of the melt measured a pH of about 7.

About 50 grams (about 1.25 'moles) of NaOH were melted at 355 C. andheld thereat for about 2% hours while N0 at a flow rate of 130cc./minute at room temperature and a partial pressure of about oneatmosphere was passed into the melt. After that period of reaction theefi luent gas exhibited a brown coloration which infrared analysisindicated to be N0 A calculation similar to that described above withregard to the reaction of N0 and KOH also determined an approximate 1:1molar reaction ratio of NaOH and N0 Infrared analysis of the solidreaction product reported the presence of nitrate and nitrite.

- Having discovered that S0 can be removed from a gas by bubblingthrough a bath of molten nitrate and N0 can be removed from a gas bybubbling through a bath of molten hydroxide, an equimolar melt of NaNOand NaOH (42.5 grams NaNO +20 grams NaOH) was melted at 400 C. and S0 ata flow rate of 120 cc. per minute and a partial pressure of about oneatmosphere passed into the melt.After hour at 400 C., the melt had builtup sufiicient sulfate to cause the mixture to freeze and a slight browncoloration was seen in the eflluent gas. Infrared analysis of thisefiluent gas indicated the presence of N0 and S0 Assuming that theoverall reaction taking place is contemplated in the following equation:

it was apparent that the stoichiometry thereof requires a ratio of twomoles hydroxide to one mole of nitrate. Therefore, an excess of nitrate,as would be present in the equimolar melt, led to the production of moreN0 than could be removed by the hydroxide.

Thereupon, a 66 mole percent KOH-33 mole percent KNO mixture (28 gramsKOH+25 grams KNO was melted and maintained at 500 C. S0 at a flow rateof about 120 cc./rninute and a partial pressure of about one atmospherebubbled through the melt. Infrared spectra observed soon after thebeginning of the reaction in-' dicated essentially no S0 or N0 presenttherein. However, after about an hour of reaction time infrared analysesof the effiuent gas reported the substantial pressure of S0 with some N0A considerable amount of solid material, analyzed as K 80 and NaNO wasdeposited in the reaction vessel. 1 r

Therefore, from these examples, it is believed apparent that thisinvention provides effective means for removing S0 and N0 from fluegases and other sources of air pollution. Hence, where N0 alone ispresent, its elimination can be secured through reaction with moltenNaOH and/or KOH. Where S0 is present alone or in combination with N0reaction thereof with sucessive sources of molten NaNO KNO and/or AgNOand NaOH and/or KOH will accomplish its removal or a single sourcecomprising a combination of nitrate and hydroxide will likewise beeifective. In large scale removal of these toxic gases, the use of aseries of baths (the so-called cascade-type process) might be better employed such that cleaning of the gas could continue while a spentbathwas being rechanged. v t

For fuel economies and to avoid thermal decomposition of the melts, thebath temperatures should preferably range between about 325 525 C.; itbeing appredated that the speed of reaction varies exponentially withtemperature. Inasmuch as the rates of reaction are very rapid, the flowof gas into the bath must be fast enough to prevent the buildup of solidreaction products across the entry duct thereby stopping the flow of gastherethrough: The maximum flow rate is dependent upon such factors asexcessive turbulence caused in the melt, the carryover of solid reactionproduct into the exit duct due to the excessive turbulence, incompletereaction of the gas with the melt, etc. Nevertheless, the proper rate ofgas flow can be readily determined empirically and is believed to bewell within the technical competence of one of the ordinary skill in theart.

Finally, it can be appreciated that this invention can also removecarbon dioxide (CO from a gaseous environment according to the followingreaction:

wherein M=K and/or Na. Therefore, whereas CO is not generally classifiedas having the toxicity of S0 or N0 it does possess harmfulcharacteristics and this invention does have the capability of removingit from the atmosphere.

. To illustrate this reaction, which is also very rapid, 50 grams (about0.9 mole) of KOH were melted and maintained at 440 C. CO at a flow rateof about cc./ minute at room temperature and a partial pressure of 0.94atmosphere was passed into the melt. Employing infrared analyses tomonitor the reaction, it was observed that essentially no CO was presentin the efiluent gas until about 1 hour after the reaction had begun withwater vapor constituting the released gas. Analysis of the reactionproduct indicated the substantial conversion of KOH to K CO A check ofthe stoichiometry of the reaction utilizing the resultant weight changedetermined that one mole of K CO was formed from two miles of KOH.

We claim:

1. A method for removing sulfur dioxide and/or nitrogen dioxide and/orcarbon dioxide from a gaseous environment which comprises passing saidgaseous environment through (1) a bath of a molten nitrate selected fromthe group consisting of NaNO AgNO and KNO and then through a bath of amolten hydroxide selected from the group consisting of NaOH and KOH; or(2) a bath composed of a combination of at least one molten nitrateselected from the group consisting of NaNO;,, AgNO and'KNO and at leastone molten hydroxide selected from the group consisting of NaOH and KOH.

2. A method according to claim 1 wherein said baths are maintained at atemperature between about 325 3. A method according to claim 1 whereinsaid combination bath consists of about 66 mole percent hydroxide and 33mole percent nitrate.

References Cited UNITED STATES PATENTS 3,438,722 4/1969 Heredy et al23-2 OTHER REFERENCES A.P.C. application of Beck et a1. Ser. No. 292,742, published July 13, 1943 (abandoned).

A.P.C. application of Beck et al., Ser. No. 393,258, pulished July 13,1943 (abandoned).

EARL C. THOMAS, Primary Examiner US. Cl. X.R.

7/ 1935 Rosenstein 23-102

